This invention relates to a method and apparatus for extending the life of a thermowell tube positioned in a hot gas duct to contain a thermocouple capable of measuring the temperature of a hot gas in the duct.
The use of burners positioned to generate hot gas for passage through a hot gas duct to heat water or other substances contained in heat exchanger tubes positioned in the duct is well known and is a widely used technique for generating steam and the like. Frequently, the gases passed through such hot gas ducts are at temperatures of up to at least 1100xc2x0 F. to about 1800xc2x0 F. It is desirable to measure the temperatures of these gases reliably since excessive temperature can damage the heat exchange tubes and the like. These temperature measurements are frequently made by positioning thermowell tubes across the height of the duct to contain a thermocouple, which then senses the temperature of the hot gas flowing through the duct. In many instances, these tubes are suspended from the top of the duct and extend to a bottom restraint in the bottom of the duct so that the tube is restrained at both the top and the bottom of the tube. The tube is typically slideably restrained at the bottom of the duct so that it may thermally expand and contract.
In the use of such thermowell tubes it has been found that when temperatures above about 1200xc2x0 F. are used, the thermowell tubes degrade thermally over relatively short periods, i.e., typically one year or less. This degradation is the result of a number of factors, but a significant factor is the fact that elevated temperatures in the range of up to 1100xc2x0 F. to 1800xc2x0 F. are well above the creep range for even high temperature stainless steels and the like. As a result, it has been necessary to replace such thermowell tubes relatively frequently.
Since the replacement of these tubes is relatively expensive and since it results in an interruption of operations, methods have been sought to extend the life of such thermowell tubes.
It has now been found that the life of such thermowell tubes can be extended by the use of an extended life thermowell tube comprising: (a) a thermowell tube adapted to contain a thermocouple and having a top and a bottom, and an upstream side and a downstream side relative to a gas flow past the thermowell tube; (b) a surface near the thermowell tube bottom positioned to engage a restraining surface on a gas duct containing at least a major portion of the thermowell tube; and, (c) a thermowell stiffener positioned on the downstream side of the thermowell tube.
The invention further comprises; a method for extending the life of a thermowell tube having a top and a bottom and a downstream side relative to a hot gas flow through a hot gas duct and positioned in the hot gas flow in the hot gas duct to contain a thermocouple capable of measuring the temperature of the hot gas flow in the hot gas duct; the thermowell tube being fixed at its top and its bottom relative to the hot gas flow past the thermowell tube; the method comprising positioning a thermowell tube stiffener on the downstream side of the thermowell tube.